Pharmaceutical forms in which medication is delivered at a slow rate may be found in a plurality of polymer-based systems which enable optimization of their effects. Among these systems, some are constituted by a polymeric matrix in which a medication is dispersed in a solid state. When the matrix is subject to dissolution, the medication is gradually released in accordance with a kinetic effect which is characterized by a gradual decrease of the delivery rate. In order to maintain a system which is well adapted to the needs of a patient, it is convenient to modify the rate in a manner that medication is delivered at a constant speed: this is called a "zero-order" delivery rate.
Systems which operate by diffusion are known. In some systems, called reservoirs, diffusion through a membrane is the limiting factor in the delivery process. These systems provide a constant delivery rate if the concentration inside the membrane does not change. In other systems, called matrices, the medication in solid state is dispersed uniformly in a polymeric material. These matrices are said to be non-porous when the medication diffuses through the polymer, in which case the kinetic effect is largely dependent upon the properties of the polymer and the medication concentration. Such systems are said to be homogeneous or heterogeneous whether the solution is in a dissolved state or in a solid state. A porous matrix is also a mean of obtaining a slow release using polymers. The medication is dispersed in solid state or in solution in the pores of the matrix and diffusion to the outside is accomplished through the pores. In these cases, the polymer acts as a carrier and does not interfere with the delivery process.
Matrix forms offer various advantages, one of which is the low possibility of a sudden delivery in the case of damage to the matrix. Also, at the initial stage of dissolution, there is a decrease in latency time and/or in high delivery rate (burst effect). Other advantages are a higher mechanical resistance and a more economical manufacturing method. In the case of porous matrices, the presence of pores enables the delivery of large molecules within a reasonable time; this is particularly useful in the cases of polypeptides.
In recent years, various different approaches have been developed in order to modify the kinetics of delivery of porous matrices with a view to reaching a zero-order delivery rate.
The most common approach consists in modifying the geometry of the system. Since the dissolution of the medication entails a gradual increase of the diffusion path, this can be compensated by a well defined geometry which consists in having a dissolution surface which increases with time. Thus, the quantity of medication being released increases gradually.